On Transpiration Seasonality (Invited Presentation)

Water is the principal regulator in biosphere-atmosphere interactions. High-frequency observations at a steep hillslope in the Mediterranean climate of northern California show that different proximate evergreen tree species have very different transpiration seasonality. A surprise is that Pacific Madrones show maximal daily transpiration in the dry summer season, with concomitant impacts on local energy and CO2 exchange. We hypothesize that the tree roots at the site have access to a deep store of water, as the water table some 20 meters below the surface exhibits very dynamic fluctuations with every rain storm. We further hypothesize that deep water stores accessible to deep roots are not unique to the research site, and could explain differential resilience to droughts and insect infestations across a landscape. Here we will discuss the data requirements and strategies for upscaling the heterogeneous structure and function to a region or continent.